JPS649381B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for obtaining a hot-rolled duplex stainless steel strip that is excellent in corrosion resistance and also has good toughness and ductility as it is hot-rolled. <Industrial Application Fields> In recent years, metal materials with stable corrosion resistance against seawater have been developed for use in seawater heat exchangers, seawater structures, etc., mainly in fields such as the chemical industry, ships, and electric power. Demand has been showing a tendency to increase. Against this background, duplex stainless steel, which exhibits extremely excellent corrosion resistance in a highly corrosive chloride environment, has come into wide use. <Prior Art> Conventionally, two-phase stainless steel sheets exhibiting two phases, a ferrite phase and an austenite phase, at room temperature have been produced by hot rolling and then coiling at a temperature of 600°C or higher. It was customary to manufacture the steel by applying heat treatment or cold rolling followed by heat treatment. This is because the desired corrosion resistance, toughness, and ductility could not be achieved with hot rolling. <Purpose of the Invention> The present inventors have omitted the heat treatment in the duplex stainless steel plate manufacturing process as described above, and produced duplex stainless steel that has excellent performance such as high corrosion resistance, high toughness, and high ductility by only hot rolling. In order to obtain a steel plate, we investigated the steel composition, hot rolling conditions, etc. from various viewpoints, and as a result, we obtained the knowledge shown below. <Knowledge> Steel that has been adjusted to a specific chemical composition by restricting C, P, S, etc. is used, and after hot rolling, it is immediately heated to an extremely low temperature range that is unthinkable in the past. If the steel is rapidly cooled to a temperature of 100% and then coiled in this low temperature range, the precipitation of Cr carbides and σ phase, which tend to appear during the cooling process after hot rolling or during slow cooling after coiling, is suppressed. A hot rolled duplex stainless steel strip having excellent corrosion resistance, high toughness and high ductility can be obtained. <Structure of the Invention> This invention was made based on the above findings, and includes: C: 0.05% or less (hereinafter, % representing the component ratio is expressed as weight %), Si: 2.0% or less, Mn: 2.0%. Below, P: 0.03% or less, S: 0.015% or less, Cr: 16.0-30.0%, Ni: 3.0-9.0%, Mo: 0.2-5.0%, N: 0.45% or less, and if necessary, so ．． A: 0.05% or less, Cu: 2.0% or less, Ca: 0.0100% or less, rare earth elements: 0.10% or less, Zr: 0.10% or less, and the remainder: Fe and other unavoidable impurities. After hot-rolling steel with the following chemical composition, immediately quench it at a cooling rate of 5℃/sec or higher to 550℃.
This method is characterized in that a hot-rolled duplex stainless steel strip having excellent corrosion resistance, toughness, and ductility can be manufactured at low cost by winding at the following temperature. Next, in the method of the present invention, the reason for numerically limiting the amount of the steel composition and the hot rolling/coiling conditions as described above will be explained. A Compositional component (a) C C is an impurity element that is preferably as small as possible because it precipitates as carbide in steel and deteriorates not only the toughness and ductility of the steel material, but also the corrosion resistance. be. And its content is 0.05%
If it exceeds, even if quenching is performed after hot rolling and low-temperature winding is performed, the precipitation of the carbides cannot be sufficiently suppressed, and the desired properties in terms of toughness, ductility and corrosion resistance cannot be obtained. The C content was set at 0.05% or less. (b) Si Si is a desirable element that has a deoxidizing effect on steel and also improves corrosion resistance, but if its content exceeds 2.0%, it will impede weldability and workability. The content was set at 2.0% or less. Although the Si component exhibits the above characteristics even in a very small amount, it is preferably contained in an amount of at least 0.10% or more in order to ensure a more significant effect. (c) Mn The Mn component is also a useful element that has a deoxidizing effect on steel, and is also a desirable component that also has the effect of improving the hot workability of steel, but if its content exceeds 2.0% Because it deteriorates the corrosion resistance of steel,
The Mn content was set at 2.0% or less. The desired effect can be obtained by adding a small amount of Mn component, but
It is preferable to ensure a content of 0.50% or more. (d) P Since P impairs the weldability and hot workability of steel, it is an impurity element that is preferably as small as possible. In particular, if the P content exceeds 0.03%, the above-mentioned adverse effects become significant, so the P content was set at 0.03% or less. (e) S S is an impurity element that is preferably as small as possible since it deteriorates the corrosion resistance, ductility, and toughness of steel. In particular, if the S content exceeds 0.015%, the above-mentioned adverse effects become significant, so the S content was set at 0.015% or less. (f) Cr The Cr component is an extremely important element that improves the corrosion resistance of steel, and it is essential to add 16.0% or more in order to impart satisfactory corrosion resistance to seawater-resistant steel. On the other hand, if the content exceeds 30.0%, the workability and weldability of the steel will deteriorate, and
In order to obtain a two-phase structure, it is necessary to increase the expensive Ni content, so the Cr content was determined to be 16.0 to 30.0%. (g) Ni Ni component is an essential component for imparting high toughness to steel and increasing corrosion resistance, and its content is 3.0%.
If it is less than that, desired toughness and corrosion resistance cannot be ensured. On the other hand, if its content exceeds 9.0%,
The Ni content was determined to be 3.0 to 9.0% because the above effect would become saturated and cause an economic disadvantage. (h) Mo The Mo component has the effect of greatly improving the seawater resistance of steel, but if the content is less than 0.2%, the desired effect cannot be obtained; on the other hand, if the content exceeds 5.0%, the desired effect cannot be obtained. Since no further improvement effect could be obtained even if Mo was added, the Mo content was set at 0.2 to 5.0%. (i) N The N component has a remarkable effect of increasing the corrosion resistance of steel even when added in a small amount of 0.0100%, which is close to an impurity. Since it is difficult to dissolve N as a solid solution in excess of this amount, the N content was set at 0.45% or less. (j) so. A, Cu, Ca, rare earth elements, Zr One or more of these components may be added as necessary to further improve the toughness, hot workability, or corrosion resistance of the steel.
The reason why the amount of each element added is limited will be explained below. so. A so. Component A is added to deoxidize the steel and further improve its toughness, but if its content exceeds 0.05%, these effects will be saturated, so so. The A content was set at 0.05% or less. Cu Cu component is added to further improve the corrosion resistance of steel, but its content is 2.0%.
Since this effect becomes saturated when the Cu content exceeds 2.0%, the Cu content was set at 2.0% or less. Ca, rare earth elements, and Zr One or more of these components are added to improve the hot workability of steel, and the Ca content is 0.0100% and the rare earth element content is 0.10%.
And if the Zr content exceeds 0.10%, the toughness will deteriorate, so the Ca content is 0.0100%, the rare earth element content is 0.10% or less,
The Zr content was set at 0.10% or less. B Hot rolling/coiling conditions (a) Cooling rate If the cooling rate after hot rolling is slower than 5°C/sec, carbides and σ phase will precipitate during cooling, which will affect the toughness, ductility, and corrosion resistance of the steel. This results in a significant deterioration of the Therefore, the cooling rate after hot rolling was determined to be 5° C./sec or more. (b) Coiling temperature If the coiling temperature is higher than 550℃, carbides and σ phase will precipitate during slow cooling after coiling, even if the cooling rate after hot rolling is 5℃/sec or more. In particular, since the toughness, ductility, and corrosion resistance of the steel would be significantly degraded, the coiling temperature was set at 550°C or higher. In addition, if the hot rolling is done at a high temperature of 830℃ or higher, the warm working structure of ferrite will be reduced, and the ductility and toughness of the steel will further improve, and if the coiling temperature is adjusted to 500 to 200℃, Precipitation of carbides and σ phase is almost completely suppressed, and the warm-worked structure of ferrite is also given the opportunity to be tempered during slow cooling after winding, making it possible to achieve even better properties and ductility. It becomes possible. Therefore, hot rolling is finished at a high temperature of 830°C or higher and then rapidly cooled to a temperature of 500 to 200°C.
Winding at ℃ is recommended. Figure 1 shows 0.011%C-0.75%Si-1.65%Mn-0.014%P, which is the target steel for the method of the present invention.
−0.0005%S−22.1%Cr−5.1%Ni−0.3%Mo−
This is a graph showing the influence of hot rolling finishing temperature and coiling temperature on the strength, ductility, toughness, and corrosion resistance of 0.142%N steel (〇 marks finish at 900°C, ● marks finish at 800°C.
The finished product is shown), and the hot rolling heating temperature is
The temperature was 1200℃ and the finished plate thickness was 5.5mm.
In addition, the cooling rate between coiling after hot rolling is 5 to 45 in all cases.
The corrosion loss is within the range of °C/sec.
This was measured by conducting a seawater immersion field test (6 months). It is clear from FIG. 1 that keeping the coiling temperature below 550°C is an extremely important factor in improving the corrosion resistance and mechanical properties of steel. Next, the present invention will be specifically explained using examples and comparing with comparative examples. <Example> First, steels A to K having the compositions shown in Table 1 were produced by a conventional method. Next, each of these steels was hot rolled under the conditions shown in Table 2 and coiled to produce a hot rolled steel strip having a thickness of 5.5 mm. Test pieces were cut out from each of the hot rolled steel strips thus obtained, and their mechanical properties and corrosion resistance were examined, and the same results as shown in Table 2 were obtained. As is clear from the results shown in Table 2,

【table】

【table】

[Table] All hot-rolled steel strips obtained by the method that satisfies the conditions of the present invention exhibit excellent mechanical properties and corrosion resistance as hot-rolled, whereas coiling starts after the end of hot rolling. The hot-rolled steel strips obtained by the methods of Test Nos. 1 and 2, in which the cooling rate and coiling temperature are outside the conditions of the present invention, have poor ductility, toughness, and corrosion resistance as they are hot-rolled. It can be seen that it is significantly inferior. Of course, the hot-rolled steel strip obtained by the method of the present invention can be used as hot-rolled to obtain fully satisfactory effects, but it has excellent toughness and ductility, and can be used as a coil. There is no problem of cracking or breaking during unwinding or molding.
Furthermore, even when used after heat treatment or cold rolling, handling is extremely easy and high quality products can be manufactured with good yield. Furthermore, it goes without saying that this method is also effective for manufacturing hot-rolled coils of two-phase stainless steel cladding. <Overall Effects> As described above, according to the present invention, two sheets of steel with excellent corrosion resistance, toughness, and ductility can be obtained even after hot rolling.
Compatible stainless steel strips can be produced stably at low cost, and can exhibit excellent performance when used as materials for heat exchangers that use seawater, chemical manufacturing equipment, salt manufacturing equipment, etc. Industrially useful effects such as these are brought about.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the influence of hot rolling finishing temperature and coiling temperature on the strength, ductility, toughness and corrosion resistance of duplex stainless steel.

Claims (1)

[Claims] 1 In terms of weight percentage, C: 0.05% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.03% or less, S: 0.015% or less, Cr: 16.0 to 30.0%, Ni : 3.0 to 9.0%, Mo: 0.2 to 5.0%, N: 0.45% or less, and the remainder is Fe and other unavoidable impurities. Immediately after hot rolling, the steel is heated at 5°C/sec. Perform rapid cooling at a cooling rate of 550
A method for producing a high-toughness duplex stainless steel hot-rolled steel strip, the method comprising coiling at a temperature of 0.degree. C. or less. 2 In terms of weight percentage, C: 0.05% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.03% or less, S: 0.015% or less, Cr: 16.0 to 30.0%, Ni: 3.0 to 9.0%, Contains Mo: 0.2 to 5.0%, N: 0.45% or less, and further contains so. A: A steel containing 0.05% or less, with the remainder consisting of Fe and other unavoidable impurities, is hot-rolled and immediately quenched at a cooling rate of 5°C/sec or higher to produce a 550%
A method for producing a high-toughness duplex stainless steel hot-rolled steel strip, the method comprising coiling at a temperature of 0.degree. C. or less. 3 In terms of weight percentage, C: 0.05% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.03% or less, S: 0.015% or less, Cr: 16.0-30.0%, Ni: 3.0-9.0%, After hot rolling a steel containing Mo: 0.2 to 5.0%, N: 0.45% or less, Cu: 2.0% or less, and the remainder consisting of Fe and other unavoidable impurities. , immediately perform rapid cooling at a cooling rate of 5℃/sec or higher to 550℃.
A method for producing a high-toughness duplex stainless steel hot-rolled steel strip, the method comprising coiling at a temperature of 0.degree. C. or less. 4 In terms of weight percentage, C: 0.05% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.03% or less, S: 0.015% or less, Cr: 16.0-30.0%, Ni: 3.0-9.0%, Contains Mo: 0.2 to 5.0%, N: 0.45% or less, and further contains one or more of the following: Ca: 0.0100% or less, rare earth elements: 0.10% or less, Zr: 0.10% or less, and the rest is After hot rolling a steel having a composition consisting of Fe and other unavoidable impurities, it is immediately quenched at a cooling rate of 5°C/sec or more, and coiled at a temperature of 550°C or less. A method for producing a high toughness duplex stainless steel hot rolled steel strip. 5 In terms of weight percentage, C: 0.05% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.03% or less, S: 0.015% or less, Cr: 16.0 to 30.0%, Ni: 3.0 to 9.0%, Contains Mo: 0.2 to 5.0%, N: 0.45% or less, Cu: 2.0% or less, Ca: 0.0100% or less, rare earth elements: 0.10% or less, Zr: 0.10% or less, and one of the following. After hot rolling a steel containing the above, with the remainder consisting of Fe and other unavoidable impurities, it is immediately quenched at a cooling rate of 5°C/sec or more, and then rolled at a temperature of 550°C or less. A method for producing a high-toughness duplex stainless steel hot-rolled steel strip, the method comprising: